1. Technical Field
The present invention relates to radio transmitters, in particular, a radio transmitter that requires fluctuation adjustment for transmission output power.
2. Related Art
Typically, radio transmitters (hereinafter, simply referred to as transmitters) have been used in various fields including the communication field. One example of these radio transmitters allows fluctuation adjustment for transmission output power in order to restrict a communication area. Such a transmitter is used in a so-called passive entry system, a tire pneumatic pressure monitoring system and the like for vehicles such as an automobile (hereinafter, simply referred to as vehicles).
The passive entry system is configured with an in-vehicle device and a portable device whereas the tire pneumatic pressure monitoring system is configured with an in-vehicle device and a sensor unit. In the passive entry system, the in-vehicle device transmits, to the portable device, response request signals (e.g., a signal for activating the portable device, a signal for requesting transmission of an ID number). In the tire pneumatic pressure monitoring system, likewise, the in-vehicle device transmits response request signals to the sensor unit. In any of the systems, the in-vehicle device receives a response signal from the portable device or the sensor unit to perform a required operation based on the response signal. More specifically, the in-vehicle device in the passive entry system performs locking or unlocking of a door. On the other hand, the in-vehicle device in the tire pneumatic pressure monitoring system determines whether a tire has a satisfactory pneumatic pressure and then gives a warning if the pneumatic pressure is unsatisfactory.
Each of the in-vehicle devices in these systems is a “transmitter” that allows fluctuation adjustment for transmission output power. The reason that the in-vehicle device requires the fluctuation adjustment for transmission output power is described below. In the passive entry system, the in-vehicle device must grasp a position of the portable device, that is, must determine whether the portable device is present inside or outside a vehicle. If the in-vehicle device fails to grasp the position of the portable device, there is a possibility that the in-vehicle device locks a door with the portable device being left inside the vehicle. In the tire pneumatic pressure monitoring system, moreover, the in-vehicle device must establish communications with the sensor unit attached to each tire. Herein, excessively large transmission output power causes disadvantageous communications between the in-vehicle device and the sensor unit other than the target sensor unit. On the other hand, excessively small transmission output power causes failure of appropriate communications between the in-vehicle device and the target sensor unit. In order to solve these problems, the transmission output power described above must be optimized for each model of vehicles. Consequently, variations in shape of vehicles cause a change in optimal transmission output power.
In the tire pneumatic pressure monitoring system, further, the in-vehicle device does not necessarily grasp a position of the sensor unit, but requires adjustment for transmission output power in order to establish communications with only the target sensor unit as in the case of the in-vehicle device in the passive entry system. It is assumed herein that the tire pneumatic pressure monitoring system is applied to a four-wheel vehicle. In this case, the sensor units are attached to left and right front tires, left and right rear tires, and a spare tire, respectively. Further, antennas are provided on tire housings for the left and right front wheels, tire housings for the left and right rear wheels, and a storage section (e.g., a trunk room) for the spare tire. Herein, these antennas are driven in turn to transmit a response request signal from the in-vehicle device to the corresponding sensor units. In a case where the antenna provided on the tire housing for the left front wheel transmits a response request signal with appropriate transmission output power, probably, only the sensor unit attached to the left front tire transmits a response signal to the antenna in a stable manner. However, if the transmission output power is excessively large, there is a possibility that the sensor unit for the different tire (the right front tire in this case) erroneously makes a response. Alternatively, if the transmission output power is excessively small, there is a possibility that the sensor unit for the target tire (the left front tire in this case) fails to make a response. In order to solve these problems, actually, tests are conducted by trial and error while adjusting fluctuation of the transmission output power in order to find the appropriate transmission output power. These tests must be conducted for each model of vehicles because a size of a tire, a shape of a tire housing, and the like are unique to each model.
As a conventional technique about fluctuation adjustment for transmission output power, for example, there has also been known that disclosed in JP 2006-174154A. In summary, this conventional technique adopts (1) a method of changing a power supply voltage of a transmitting part, (2) a method of changing a transmission frequency and (3) a method of changing a duty of a rectangular waveform of a transmission signal (i.e., a ratio of occupation by ON periods in one cycle length of a waveform) singly or in combination to achieve the fluctuation adjustment for transmission output power.
According to the method (1), fluctuation adjustment for transmission output power is performed in such a manner that an amplitude of a transmission signal is changed. According to the method (2), moreover, fluctuation adjustment for transmission output power is performed by use of a frequency response characteristic of a transmission antenna of a transmitter in a vehicle and that of an antenna of a receiver (e.g., the portable device or the sensor unit in the example described above). According to the method (3), further, fluctuation adjustment for transmission output power is performed in accordance with a ratio of occupation by ON periods in one cycle length of a waveform.
However, this conventional technique has the following disadvantage.
In the method (2), a reception sensitivity depends on a frequency response characteristic of a reception antenna. Consequently, an antenna having a high Q value causes a significant change in sensitivity only when a frequency varies slightly. For this reason, the sensitivity can be adjusted with good accuracy only in a range where the reception sensitivity does not vary largely around a specified frequency (typically 125 KHz). Further, the adjustment for the sensitivity becomes more difficult in a case where a frequency response characteristic of an antenna is unique to each product.